The present invention pertains in general to methods for multiple string, thermal fluid injection and in particular to parallel string, thermal fluid injection methods.
An oil-producing well may pass through several petroleum containing strata. These strata may differ in permeability, homogeneity and thickness. Furthermore, the petroleum in these strata may differ in amount, viscosity, specific gravity and average molecular weight.
Where petroleum within a stratum is so viscous that the temperature and pressure within the stratum are insufficient to cause it to flow to a producing well, hot fluids, particularly steam, are injected into such strata in order to raise the temperature of the stratum and thereby reduce the viscosity of the petroleum contained therein to a point at which the petroleum flows to a producing well bore.
In some wells, it is desirable to treat more than one stratum with hot fluids. Where these strata require different injection techniques, which may include the use of fluids at different temperatures (hereinafter "thermal fluid injection") and different pressures, separate conduction pathways are used for each different type of fluid.
Commonly, for thermal fluid injection, metallic steam injection tubing is run into wells which have been drilled and cased. Packers are placed between the tubing and the casing above and sometimes below the stratum to be injected. Next, the wellhead is connected to a source of hot fluid, such as a steam generator. The hot fluid is pumped into the stratum formation through the tubing.
Two types of tubing strings have been used or suggested for simultaneous thermal fluid injection into more than one stratum.
In the first type, concentric tubing strings of the sort shown in U.S. Pat. No. 4,399,865, are formed by running a first steam-bearing pipe within a second to form two flow channels. In the second type, a multichannel conduit, of the sort shown in U.S. Pat. No. 4,424,859, is composed of a plurality of contiguous flow channels within a cylindrical shell.
Both of these types of tubing string suffer from a severe problem of heat transfer between flow channels. Uninsulated channels in either of these two types of strings act as heat exchangers and thereby reduce the efficiency of any attempt to inject fluids at different temperatures into separate strata. Insulated flow channels, although more thermally effective, have size and cost disadvantages.